U.S. patent number 7,713,859 [Application Number 11/463,355] was granted by the patent office on 2010-05-11 for tin-silver solder bumping in electronics manufacture.
This patent grant is currently assigned to Enthone Inc.. Invention is credited to Joseph A. Abys, Marlies Kleinfeld, Thomas B. Richardson, Christian Rietmann, Ortrud Steinius, Igor Zavarine, Yun Zhang.
United States Patent |
7,713,859 |
Richardson , et al. |
May 11, 2010 |
Tin-silver solder bumping in electronics manufacture
Abstract
A process for forming a solder bump on an under bump metal
structure in the manufacture of a microelectronic device comprising
exposing the under bump metal structure to an electrolytic bath
comprising a source of Sn.sup.2+ ions, a source of Ag.sup.+ ions, a
thiourea compound and/or a quaternary ammonium surfactant; and
supplying an external source of electrons to the electrolytic bath
to deposit a Sn--Ag alloy onto the under bump metal structure.
Inventors: |
Richardson; Thomas B.
(Killingworth, CT), Kleinfeld; Marlies (Wuppertal,
DE), Rietmann; Christian (Sonsbeck, DE),
Zavarine; Igor (New Haven, CT), Steinius; Ortrud
(Wuppertal, DE), Zhang; Yun (Warren, NJ), Abys;
Joseph A. (Warren, NJ) |
Assignee: |
Enthone Inc. (West Haven,
CT)
|
Family
ID: |
37743065 |
Appl.
No.: |
11/463,355 |
Filed: |
August 9, 2006 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070037377 A1 |
Feb 15, 2007 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60708328 |
Aug 15, 2005 |
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Current U.S.
Class: |
438/613;
257/E23.026; 257/E21.508; 257/779; 257/772 |
Current CPC
Class: |
C25D
3/60 (20130101); H01L 21/2885 (20130101); H01L
24/11 (20130101); H01L 24/12 (20130101); H01L
2924/01033 (20130101); H01L 2224/11462 (20130101); H01L
2924/01057 (20130101); H01L 2924/01047 (20130101); H01L
2224/13099 (20130101); H01L 2924/014 (20130101); H01L
2924/01013 (20130101); H01L 2924/01016 (20130101); H01L
2924/01023 (20130101); H01L 2924/01078 (20130101); H01L
2924/01041 (20130101); H01L 2924/01029 (20130101); H01L
2924/01322 (20130101); C25D 21/18 (20130101); H01L
2924/01006 (20130101); H01L 2924/01022 (20130101); H01L
2924/01082 (20130101) |
Current International
Class: |
H01L
33/62 (20100101) |
Field of
Search: |
;257/E21.508,772,779,E23.026 ;438/613 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
International Search Report, PCT/US2006/031618, dated Jan. 25,
2008, 3 pages. cited by other .
Written Opinion, PCT/US2006/031618, dated Jan. 25, 2008,8 pages.
cited by other .
International Preliminary Report on Patentability,
PCT/US2006/031618, dated Mar. 17, 2009, 7 pages. cited by other
.
Trimble, Russell, "Comparison of the Reactions of Amidinourea and
Amidinothiourea with Various Metal Ions", Analytical Chemistry,
vol. 34, No. 12, Nov. 1962, pp. 1633-1635. cited by other .
De Marco et al., "Thermodynamics of Complex-Formation of Ag(I) Part
10. Investigations on Complex Equilibria Between Ag(I) and
Thioureas in Ethanol", Thermochimica Acta, 1994, vol. 246 No. 1,
pp. 229-242. cited by other .
European Search Report, European Patent Application No. 06801412.5,
Dec. 4, 2009, 12 pages. cited by other.
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Primary Examiner: Phung; Anh
Assistant Examiner: Lulis; Michael
Attorney, Agent or Firm: Senniger Powers LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit, of U.S. Provisional
Application Ser. No. 60/708,328, filed Aug. 15, 2005.
Claims
What is claimed is:
1. An electroplating composition for plating a Sn--Ag solder bump
comprising: a source of Sn.sup.2+ ions; a source of Ag.sup.+ ions;
a N-allyl-thiourea compound having a formula (I): ##STR00005##
wherein R.sub.2, R.sub.3, R.sub.4, R.sub.5, and R.sub.6 are each
independently hydrogen, hydroxyl, hydrocarbyl, substituted
hydrocarbyl, heterocycloalkyl, substituted heterocycloalkyl,
alkoxy, substituted alkoxy, aryl, substituted aryl, heteroaryl, or
substituted heteroaryl and R.sub.1 is --CH.sub.2CH.sub.2OH
(.beta.-hydroxyethyl); and a quaternary ammonium surfactant.
2. A process for forming a solder bump on an under bump metal
structure in the manufacture of a microelectronic device, the
process comprising: exposing the under bump metal structure to an
electrolytic bath comprising a source of Sn.sup.2+ ions, a source
of Ag.sup.+ ions, and an N-allyl-thiourea compound having a formula
(I): ##STR00006## wherein R.sub.2, R.sub.3, R.sub.4, R.sub.5, and
R.sub.6 are each independently hydrogen, hydroxyl, hydrocarbyl,
substituted hydrocarbyl, heterocycloalkyl, substituted
heterocycloalkyl, alkoxy, substituted alkoxy, aryl, substituted
aryl, heteroaryl, or substituted heteroaryl and R.sub.1 is
--CH.sub.2CH.sub.2OH (.beta.-hydroxyethyl); and supplying an
external source of electrons to the electrolytic bath to deposit a
Sn--Ag alloy onto the under bump metal structure.
3. The process of claim 2 wherein R.sub.2, R.sub.3, R.sub.4,
R.sub.5, and R.sub.6 are hydrogen.
4. The process of claim 2 wherein the N-allyl-thiourea compound has
a concentration between about 0.5 g/L and about 24 g/L.
5. The process of claim 2 wherein the N-allyl-thiourea compound has
a concentration between about 1 g/L and about 8 g/L.
6. The process of claim 2 wherein the supplying the external source
of electrons employs a current density of at least about 8
A/dm.sup.2.
7. The process of claim 2 wherein the supplying the external source
of electrons employs a current density between about 8 A/dm.sup.2
and about 20 A/dm.sup.2.
8. The process of claim 2 wherein the electrolytic bath further
comprises an alkyl dimethyl benzyl ammonium surfactant.
9. The process of claim 2 wherein the electrolytic bath further
comprises a .beta.-naphthol derivative.
10. The process of claim 2 wherein the electrolytic bath further
comprises hydroquinone.
11. The process of claim 2 wherein the electrolytic bath further
comprises a quaternary ammonium surfactant and a .beta.-naphthol
derivative.
12. The process of claim 2 wherein the electrolytic bath further
comprises a quaternary ammonium surfactant, a .beta.-naphthol
derivative, and hydroquinone.
13. The process of claim 2 wherein the electrolytic bath comprises:
between about 0.5 g/L and about 24 g/L of the N-allyl-thiourea
compound; the Sn.sup.2+ ions in a concentration between about 10
g/L and about 100 g/L; and the Ag.sup.+ ions in a concentration
between about 0.1 g/L and about 1.5 g/L.
14. The process of claim 13 wherein the supplying the external
source of electrons employs a current density between about 8
A/dm.sup.2 and about 20 A/dm.sup.2.
15. The process of claim 2 wherein the electrolytic bath further
comprises a .beta.-naphtholethoxylate comprising between 8 and 16
ethylene oxide monomer units.
16. A process for forming a solder bump on an under bump metal
structure in the manufacture of a microelectronic device, the
process comprising: exposing the under bump metal structure to an
electrolytic bath comprising a source of Sn.sup.2+ ions, a source
of Ag.sup.+ ions, and an N-allyl-thiourea compound having a formula
(I): ##STR00007## wherein R.sub.2, R.sub.3, R.sub.4, R.sub.5, and
R.sub.6 are each independently hydrogen, hydroxyl, hydrocarbyl,
substituted hydrocarbyl, heterocycloalkyl, substituted
heterocycloalkyl, alkoxy, substituted alkoxy, aryl, substituted
aryl, heteroaryl, or substituted heteroaryl and R.sub.1 is
--CH.sub.2CH.sub.2OH (.beta.-hydroxyethyl), and a quaternary
ammonium surfactant; and supplying an external source of electrons
to the electrolytic bath to deposit a Sn--Ag alloy onto the under
bump metal structure.
17. A process for forming a solder bump on an under bump metal
structure in the manufacture of a microelectronic device, the
process comprising: exposing the under bump metal structure to an
electrolytic bath comprising a source of Sn.sup.2+ ions, a source
of Ag.sup.+ ions, an N-allyl-thiourea compound having a formula
(I): ##STR00008## wherein R.sub.2, R.sub.3, R.sub.4, R.sub.5, and
R.sub.6 are each independently hydrogen, hydroxyl, hydrocarbyl,
substituted hydrocarbyl, heterocycloalkyl, substituted
heterocycloalkyl, alkoxy, substituted alkoxy, aryl, substituted
aryl, heteroaryl, or substituted heteroaryl and R.sub.1 is
--CH.sub.2CH.sub.2OH (.beta.-hydroxyethyl), lauryl benzyl dimethyl
ammonium chloride and myristyl benzyl dimethyl ammonium chloride;
and supplying an external source of electrons to the electrolytic
bath to deposit a Sn--Ag alloy onto the under bump metal
structure.
18. A process for forming a solder bump on an under bump metal
structure in the manufacture of a microelectronic device, the
process comprising: exposing the under bump metal structure to an
electrolytic bath comprising a source of Sn.sup.2+ ions, a source
of Ag.sup.+ ions, and an N-allyl-thiourea compound having a formula
(I): ##STR00009## wherein R.sub.2, R.sub.3, R.sub.4, R.sub.5, and
R.sub.6 are each independently hydrogen, hydroxyl, hydrocarbyl,
substituted hydrocarbyl, heterocycloalkyl, substituted
heterocycloalkyl, alkoxy, substituted alkoxy, aryl, substituted
aryl, heteroaryl, or substituted heteroaryl and R.sub.1 is
--CH.sub.2CH.sub.2OH (.beta.-hydroxyethyl), and a sulfonated
.beta.-naphthol propoxylate/ethoxylate; and supplying an external
source of electrons to the electrolytic bath to deposit a Sn--Ag
alloy onto the under bump metal structure.
19. A process for forming a solder bump on an under bump metal
structure in the manufacture of a microelectronic device, the
process comprising: exposing the under bump metal structure to an
electrolytic bath comprising a source of Sn.sup.2+ ions, a source
of Ag.sup.+ ions, and a quaternary ammonium surfactant wherein the
quaternary ammonium surfactant comprises lauryl benzyl dimethyl
ammonium chloride and myristyl benzyl dimethyl ammonium chloride;
and supplying an external source of electrons to the electrolytic
bath to deposit a Sn--Ag alloy onto the under bump metal
structure.
20. The process of claim 19 further comprising a N-allyl-thiourea
compound.
21. The process of claim 20 wherein the N-allyl-thiourea compound
is N-allyl-thiourea.
22. The process of claim 20 wherein the N-allyl-thiourea compound
is N-allyl-N'-.beta.-hydroxyethyl-thiourea.
23. The process of claim 19 wherein the supplying the external
source of electrons employs a current density between about 8
A/dm.sup.2 and about 20 A/dm.sup.2.
24. The process of claim 19 wherein the electrolytic bath further
comprises a .beta.-naphthol derivative.
25. The process of claim 19 wherein the electrolytic bath further
comprises a .beta.-naphtholethoxylate, a sulfonated .beta.-naphthol
propoxylate/ethoxylate, or a combination thereof.
26. The process of claim 19 wherein the electrolytic bath further
comprises hydroquinone.
27. The process of claim 19 wherein the electrolytic bath further
comprises an N-allyl-thiourea compound and a .beta.-naphthol
derivative.
28. The process of claim 19 wherein the electrolytic bath further
comprises an N-allyl-thiourea compound, a .beta.-naphthol
derivative, and hydroquinone.
Description
FIELD OF THE INVENTION
This invention relates to electroplating baths and methods for
plating tin-based alloys, and more specifically, the invention
relates to tin-silver (Sn--Ag) alloy solder wafer bumping in the
manufacture of microelectronic devices.
BACKGROUND OF THE INVENTION
Solder wafer bumps conventionally comprise solders of the tin-lead
(Sn--Pb) alloy group. The Sn--Pb solders may be formed in a variety
of compositions including the low melting eutectic comprising 63%
Sn and 37% lead. High Sn alloys are called fine solders and are
used extensively in electrical work. Many Sn--Pb alloy compositions
exhibit broad pasty temperature ranges which enhance their
workability.
Recent regulatory and environmental developments have increased
interest in Pb-free solders. Accordingly, pure Sn, Sn--Cu, Sn--Bi,
Sn--Ag, and ternary Sn alloys have been explored as potential
alternatives to Sn--Pb alloys. Of particular interest are the
Sn--Ag alloys because of their performance advantages such as low
resistivity, stability, the ability to achieve a wide range of
melting points, and the elimination of alpha particle emissions by
using pure Sn sources.
A particular problem associated with the use of Ag in Sn--Ag alloy
solder wafer bumping is associated with the spontaneous reduction
of Ag ions from the electroplating bath. For example, Ag ions,
being very noble, have a tendency toward immersion/displacement
plating upon exposure to certain UBM layers, particularly Cu
layers. Accordingly, precise control of the Ag ion concentration in
the electroplating solution and therefore control of the Ag metal
content and uniformity in the Sn--Ag solder wafer bump is rendered
difficult. There is a need for a plating method which allows for
control of the Ag ion content in solution, and thus control of the
Ag metal content in the alloy solder wafer bump.
Another problem facing microelectronic device manufacturers using
Sn--Ag alloy solder wafer bumps is low throughput due to the
limited current densities which may be achieved using conventional
electroplating baths. For example, in U.S. Pat. No. 6,638,847, it
was reported that current densities appropriate for electroplating
Sn-based alloys, include Sn--Ag, were in the range of 3-5 ASD. Kim
et al. report current densities for plating a Sn--Ag solder using
thiourea as a complexing agent in the range of 1 to 3 ASD. See
Effects of Electroplating Parameters on Composition of Sn--Ag
Solder, J. Electronic Materials, December 2004. Accordingly, there
is a need for a plating composition which can plate at high current
densities to achieve higher throughput.
SUMMARY OF THE INVENTION
Among the various aspects of the present invention may be noted the
provision of electrolytic plating baths and methods for plating
Sn--Ag alloy solder wafer bumps, the baths being characterized by
additives which enhance the stability of Ag.sup.+ ions in solution,
allow for plating at high current densities, and plate Sn--Ag wafer
bumps with substantially reduced or entirely eliminated voiding
between the bumps and Cu UBM.
Briefly, therefore, the present invention is directed to a process
for forming a solder bump comprising exposing an under bump metal
structure to an electrolytic bath comprising a source of Sn.sup.2+
ions, a source of Ag.sup.+ ions, and an N-allyl-thiourea compound,
and supplying an external source of electrons to the electrolytic
bath to deposit a Sn--Ag alloy onto the under bump metal
structure.
The invention is also directed to a process for forming a solder
bump comprising exposing an under bump metal structure to an
electrolytic bath comprising a source of Sn.sup.2+ ions, a source
of Ag.sup.+ ions, and a quaternary ammonium surfactant; and
supplying an external source of electrons to the electrolytic bath
to deposit a Sn--Ag alloy onto the under bump metal structure.
The invention is also directed to a process for forming a solder
bump comprising exposing an under bump metal structure to an
electrolytic bath comprising a source of Sn.sup.2+ ions, a source
of Ag.sup.+ ions, and an amidinothiourea compound; and supplying an
external source of electrons to the electrolytic bath to deposit a
Sn--Ag alloy onto the under bump metal structure.
In another aspect the invention is directed to a process for
forming a Sn--Ag electrolytic bath for deposition of solder bumps,
the process comprising combining a source of Ag.sup.- ions with a
complexor for Ag.sup.+ ions and water to form a bath precursor
comprising Ag complex in a substantial absence of any source of
Sn.sup.+2 ions; and adding a source of Sn.sup.+2 ions to the bath
precursor comprising the Ag complex.
In another aspect the invention is directed to an electroplating
composition for plating a Sn--Ag solder bump comprising a source of
Sn.sup.2+ ions, a source of Ag.sup.+ ions, an N-allyl-thiourea
compound, and a quaternary ammonium surfactant; and also to an
electroplating composition for plating a Sn--Ag solder bump
comprising a source of Sn.sup.2+ ions; a source of Ag.sup.+ ions;
an amidinothiourea compound; and a quaternary ammonium
surfactant.
Other objects and features of the invention will be in part
apparent and in part pointed out hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A, 1B, and 1C are SEM photographs of solder bumps prepared
according to the method described in Example 11.
FIGS. 2A, 2B, and 2C are SEM photographs of reflowed solder bumps
prepared according to the method described in Example 12.
FIG. 3A shows a wafer having a patterned distribution of dies. FIG.
3B shows the measurement position of 5 wafer bumps within each
die.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
In accordance with the present invention, Sn--Ag alloy solder bumps
are deposited from an electroplating bath onto an under bump metal
structure, such as a silicon wafer substrate overlaid by
metallization layers, for use in the manufacture of a
microelectronic device. For example, Sn--Ag alloy solder bumps can
be deposited having uniform bump heights, uniform Ag content and
distribution in the bumps, and good reflow characteristics.
Advantageously, the electroplating bath exhibits good stability
with respect to Ag immersion/displacement of the Cu under bump
metal or soluble Sn anode and plates the Sn--Ag alloy at high
current densities. For example, in practice, the plating baths can
plate a Sn--Ag alloy at current densities up to 20 amps per square
decimeter (Amp/dm.sup.2, hereinafter "ASD") with substantially no
Ag displacement on Cu UBM or soluble Sn anode. Surprisingly, the
plating baths plate Sn--Ag alloy bumps with substantially reduced
or eliminated voiding between the bumps and Cu UBM. The invention
is described herein in the context of Sn--Ag alloy solder bumps on
microelectronic device substrates, which solder bumps are deposited
in patterns to yield, for example, the pattern of as-deposited
bumps shown in FIGS. 1A, 1B, and 1C. These bumps are reflowed to
yield the pattern of reflowed bumps shown in FIGS. 2A, 2B, and 2C.
However, the nature of the substrate as an electronic device
substrate is not critical to the applicability of the
invention.
The present invention stems from the discovery that certain
additives to a Sn--Ag electroplating bath increase Ag.sup.+ ion
stability and allow plating at high current densities. The
discovered additives substantially reduce the aforementioned
problem of spontaneous reduction of Ag.sup.+ ion onto unintended
surfaces, which had detracted from bath stability and uniformity in
Sn--Ag proportions in the intended deposits. In one embodiment, to
enhance Ag.sup.+ ion stability and extend the usable current
densities and thereby achieve these advantages, an N-allyl thiourea
compound which strongly complexes Ag.sup.+ ions is included in the
bath. One particular such compound is an N-allyl-N'-hydroxyalkyl
thiourea compound. In one embodiment, the alkyl group is ethyl, and
the hydroxyl group is in the .beta. position on the ethyl group.
This compound, N-allyl-N'-.beta.-hydroxyethyl-thiourea, is
hereinafter referred to as "HEAT". The N-allyl thiourea compound
which strongly complexes Ag.sup.+ ions substantially inhibits
Ag.sup.+ ion displacement onto the Cu UBM and soluble Sn anode. In
one embodiment, Ag.sup.+ ion stability is enhanced by including in
the bath an amidinothiourea compound which also strongly complexes
Ag.sup.+ ions. In one preferred embodiment, the amidinothiourea
compound is amidinothiourea.
The formation of the stable silver complex reduces the reduction
potential closer to that of Sn.sup.2+, such that the current
density is not limited by the otherwise rapid Ag.sup.+ ion
diffusion rate. Instead, the current density is limited by the
diffusion rate of Sn.sup.2+ ions, which are present typically in
such high concentrations, up to about 50 g/L to about 60 g/L or
more, that very high current densities can be employed.
Selected N-allyl thiourea compounds act to increase the solubility
of Ag.sup.+ ions in the bath by forming complexes with Ag.sup.+
ions at acidic pH and to inhibit displacement reactions between
Ag.sup.+ ions and Cu UBM or soluble Sn anode. Additionally, these
N-allyl thiourea compounds shift the Ag.sup.+ reduction potential
closer to the reduction potential of Sn.sup.2+, allowing for
plating at high current densities. Exemplary N-allyl thiourea
compounds include N-allyl thiourea, and HEAT. In selecting a
suitable N-allyl thiourea compound, compounds which are unable to
result in a water soluble complex with Ag.sup.+ ions at acidic pH
or are toxic are avoided. Based on the HSAB (hard-soft-acid-base)
concept and Pi-back bonding of sulfur-containing compounds, it is
thought that N-allyl thiourea compounds, such as HEAT, are useful
additives for forming complexes with Ag.sup.+ ions resulting in
tetrahedral complexes with bridged 2e-3 center bonding between
Ag--S--Ag. In general, suitable N-allyl thiourea compounds shown
below in the following general structure (1):
##STR00001## wherein R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5,
and R.sub.6 are each independently hydrogen, hydroxyl, hydrocarbyl,
substituted hydrocarbyl, heterocycloalkyl, substituted
heterocycloalkyl, alkoxy, substituted alkoxy, aryl, substituted
aryl, heteroaryl, or substituted heteroaryl.
Where any of R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, and
R.sub.6 are heterocycloalkyl or heteroaryl, the ring substituent
may comprise O, S, or N atoms. Exemplary substituents on the
substituted R groups include halide, hydroxyl, alkoxy, aryl,
heteroaryl, nitro, cyano, and mercaptan. In one preferred
embodiment, R.sub.2, R.sub.3, R.sub.4, R.sub.5, and R.sub.6 are
hydrogen. In another preferred embodiment, R.sub.1, R.sub.2,
R.sub.3, R.sub.4, R.sub.5, and R.sub.6 are hydrogen, and the
compound is N-allyl thiourea. In a particularly preferred
embodiment, R.sub.2, R.sub.3, R.sub.4, R.sub.5, and R.sub.6 are
hydrogen and R.sub.1 is --CH.sub.2CH.sub.2OH (.beta.-hydroxyethyl),
and the N-allyl thiourea compound is HEAT, which has the following
structure (2):
##STR00002##
Selected amidinothiourea compounds act to increase the solubility
of Ag.sup.+ ions in the bath by forming complexes with Ag.sup.+
ions at acidic pH and to inhibit displacement reactions between
Ag.sup.+ ions and Cu UBM or soluble Sn anode. In selecting a
suitable amidinothiourea compound, compounds which are unable to
result in a water soluble complex with Ag.sup.+ ions at acidic pH
or are toxic are avoided. The general formula of suitable
amidinothiourea compounds are shown below in the following formula
(3):
##STR00003## wherein
R.sub.8 and R.sub.9 are either sulfur or nitrogen, and when R.sub.8
is sulfur, R.sub.9 is nitrogen and when R.sub.8 is nitrogen,
R.sub.9 is sulfur; and
R.sub.10, R.sub.11, R.sub.12, and R.sub.13 are each independently
hydrogen, hydroxyl, hydrocarbyl, substituted hydrocarbyl,
heterocycloalkyl, substituted heterocycloalkyl, alkoxy, substituted
alkoxy, aryl, substituted aryl, heteroaryl, or substituted
heteroaryl.
Exemplary substituents on the substituted R.sub.10, R.sub.11,
R.sub.12, and R.sub.13 include halide, hydroxyl, alkyl, alkoxy,
aryl, heteroaryl, nitro, cyano, and mercaptan. In one preferred
embodiment, R.sub.10, R.sub.11, R.sub.12, and R.sub.13 are all
hydrogen, and the amidinothiourea compound is amidinothiourea,
which has the following structure (4):
##STR00004##
In the baths of the present invention, the concentration of the
N-allyl-thiourea compound or amidinothiourea compound is between
about 0.5 g/L and about 24 g/L. Employing too much of the compound
causes decomposition to free thiourea derivative. Employing too
little results in insufficient complexing of Ag. In one embodiment,
the concentration is at least about 1 g/L, such as about 1.4 g/L,
about 1.5 g/L, about 1.75 g/L, or about 2.2 g/L. In one embodiment,
the concentration is no greater than about 8 g/L, such as about 8
g/L, about 6 g/L, about 4 g/L, or about 3 g/L. In one embodiment,
between about 0.5 g/L and about 16 g/L of HEAT is added to complex
Ag.sup.+ ions and enhance their solubility, while also inhibiting
displacement reactions between Ag.sup.+ ions and Cu UBM or soluble
Sn anode. In one embodiment, between about 0.5 g/L and about 16
g/L, such as between about 1 and about 8 g/L of amidinothiourea is
added to complex Ag.sup.+ ions and enhance their solubility.
Silver immersion plating (also known as "displacement" plating)
occurs in electroplating baths spontaneously due to high nobility
of Ag.sup.+ ions. The reduction of Ag is described by the following
reaction: Ag.sup.++e.sup.-.fwdarw.Ag E.degree.=+0.80 V v. S.H.E. 1)
The positive value of E.degree. for the reaction indicates the very
positive redox potential of Ag.sup.+. Cu.sup.2+ and Sn.sup.4+ are
less noble than Ag.sup.+ as shown by the lower reduction potentials
of the following reduction reactions: Cu.sup.2++2e.sup.-.fwdarw.Cu
E.degree.=+0.34 V v. S.H.E. 2) Sn.sup.4++2e.sup.-.fwdarw.Sn.sup.2+
E.degree.=+0.15 V v. S.H.E. 3) Accordingly, upon exposure of
aqueous Ag.sup.+ ions to aqueous Sn.sup.2+ ions and/or Cu UBM,
Ag.sup.+ ions spontaneously oxidize Sn.sup.2+ ions to Sn.sup.4+
ions and Cu metal to Cu.sup.2- ions. Concurrently, Ag.sup.+ ions
are reduced to Ag metal, which may become a finely divided metal
floating in solution or deposit spontaneously on the substrate,
walls of the plating tank, or soluble Sn anode. Spontaneous
Ag.sup.+ ion displacement plating renders difficult control of the
concentration of Ag.sup.+ ions in the electroplating bath, which is
a severe limitation in the use of Ag in electroplating alloys, such
as in Sn-based solder wafer bumps. Accordingly, the thiourea
compounds, such as HEAT and amidinothiourea, are added to the
Sn--Ag electroplating bath to complex Ag.sup.+ ions and thereby
control both the solubility and stability of Ag.sup.+ ions in the
plating bath. Advantageously, HEAT and amidinothiourea also lower
the reduction potential of Ag.sup.+ ions, bringing that potential
closer to the reduction potential of Sn.sup.2+ ions, which allows
for better control of the concentration and uniformity of Ag metal
in the Sn--Ag alloy solder wafer bump and allows for plating at
high current densities.
To further enhance the effect of thiourea compounds for extending
the usable current density range, certain quaternary ammonium
surfactants are added to the bath in the currently preferred
embodiment. The quaternary ammonium surfactants which have been
found useful for adding to the plating baths of the present
invention include alkyl dimethyl benzyl ammonium salts in which the
alkyl group is between 10 and 18 carbons long, preferably the alkyl
group is 12 to 14 carbons long. The alkyl group may be substituted
or unsubstituted. A particularly preferred quaternary ammonium
surfactant is a C12/C14 dimethyl benzyl ammonium chloride salt sold
under the trade name Dodigen 226, which comprises lauryl dimethyl
benzyl ammonium chloride salt and myristyl dimethyl benzyl ammonium
chloride salt. Advantageously, this compound acts additionally as a
grain refiner and prevents nodular growth. The quaternary ammonium
surfactants further extend the useful current density range, act as
grain refiners, and inhibit nodular growth. Without being bound to
a particular theory, it is thought that the quaternary ammonium
surfactants further extend the current density range because the
salts can bind both Sn.sup.2+ and Ag.sup.+ ions, thus inhibiting
diffusion of those metal ions and allowing the plating current
density range to be extended.
In the baths of the present invention, the concentration of
quaternary ammonium surfactant is between about 0.1 g/L and about
20 g/L, preferably between about 0.8 and about 15 g/L, such as
about 0.8 g/L, about 1.6 g/L, or about 2.0 g/L. For example,
between about 0.5 g/L and about 10 g/L of Dodigen 226 may be added
to extend the useful current density range, refine the grain size,
and prevent nodular growth in the Sn--Ag solder wafer bump.
Employing too much of the compound causes bath instability and
increased nodule formation at the center of wafers. Employing too
little results in insufficient suppression of high current density
plating, and therefore increases the risk of nodule growth at high
current density areas.
In conventional Sn--Ag electroplating baths, the plating rate is
limited to low current densities. Typically, the current densities
practically achievable in conventional plating baths are no greater
than about 1 ASD. Limited current densities adversely affect
plating throughput. Conventional baths are limited to such low
current densities because of rapid diffusion rate of Ag.sup.+ ions
in the bath. The electroplating baths of the current invention
comprising thiourea compounds, such as HEAT and amidinothiourea,
and quaternary ammonium surfactant salts, such as Dodigen 226, can
be plated at current densities up to about 20 ASD. By incorporating
these additives, the increased solubility and stability of Ag.sup.+
ions in the bath largely remove Ag.sup.+ ion diffusion rate as an
overall rate limiting aspect, and makes it so Sn.sup.2+ diffusion
rate is the overall rate limiting aspect. Since the Sn.sup.2+ ion
concentration is in the range of about 50 g/L to about 60 g/L, the
diffusion rate limited current is much higher. Taking advantage of
this discovery, the process of the invention employs a current
density of greater than about 4 ASD. One preferred embodiment
employs a current density of greater than 8 ASD, and another
preferred high-throughput embodiment employs a current density of
more than 10 ASD.
Among other components of the bath may be added a source of
Sn.sup.2+ ions, a source of Ag.sup.+ ions, an anti-oxidant, and a
surfactant.
The source of Sn.sup.2+ ions may be a soluble anode comprising a
Sn.sup.2+, or, where an insoluble anode is used, a soluble
Sn.sup.2+. In both cases, the Sn.sup.2+ salt is
Sn(CH.sub.3SO.sub.3).sub.2 (Tin methane sulfonic acid, hereinafter
"Sn(MSA).sub.2") in one preferred embodiment. Sn(MSA).sub.2 is a
preferred source of Sn.sup.2+ ions because of its high solubility.
Additionally, the pH of Sn--Ag plating baths of the present
invention may be lowered using methane sulfonic acid, and the use
of Sn(MSA).sub.2 as the Sn source rather than, e.g., Sn(X), avoids
the introduction of unnecessary additional anions, e.g., X.sup.2-,
into the plating baths, especially those that render Ag.sup.+
substantially insoluble. Typically, the concentration of the source
of Sn.sup.2+ ions is sufficient to provide between about 10 g/L and
about 100 g/L of Sn.sup.2+ ions into the bath, preferably between
about 15 g/L and about 95 g/L, more preferably between about 40 g/L
and about 60 g/L. For example, Sn(MSA).sub.2 may be added to
provide between about 30 g/L and about 60 g/L Sn.sup.2+ ions to the
plating bath.
Ag.sup.+ ions are sparingly soluble with most anions. Therefore,
the source of Ag.sup.+ ions is limited to salts of nitrate,
acetate, and preferably methane sulfonate. Typically, the
concentration of the source of Ag.sup.+ ions is sufficient to
provide between about 0.1 g/L and about 1.5 g/L of Ag.sup.+ ions
into the bath, preferably between about 0.3 g/L and about 0.7 g/L,
more preferably between about 0.4 g/L and about 0.6 g/L. For
example, Ag(MSA) may be added to provide between about 0.2 g/L and
about 1.0 g/L Ag.sup.+ ions to the plating bath.
Anti-oxidants may be added to the baths of the present invention to
stabilize the bath against oxidation of Sn.sup.2+ ions in solution.
In the electroplating baths for plating Sn--Ag alloy solder wafer
bumps, Ag.sup.+ ions, being nobler than Sn.sup.2+, can
spontaneously oxidize Sn.sup.2+ to Sn.sup.4+, as explained above.
This spontaneous redox reaction can cause Ag metal to deposit on
the Cu UBM, the walls of the plating tank, or the soluble Sn anode,
and to form finely divided Ag metal particles in solution.
Additionally, reduction of Sn.sup.4+, which forms stable hydroxides
and oxides, to Sn metal, being a 4-electron process, slows the
reaction kinetics. Although HEAT and other thiourea compounds may
be added to the bath to prevent the spontaneous reduction of
Ag.sup.+/oxidation of Sn.sup.2+, it is preferable to add an
anti-oxidant to the bath to further stabilize the bath against this
redox reaction. Accordingly, preferred anti-oxidants such as
hydroquinone, catechol, and any of the hydroxyl, dihydroxyl, or
trihydroxyl benzoic acids may be added in a concentration between
about and about, preferably between about 0.1 g/L and about 10 g/L,
more preferably between about 0.5 g/L and about 3 g/L. For example,
hydroquinone may be added to the bath at a concentration of about 2
g/L.
Surfactants may be added to promote wetting of the under bump metal
structure and resist material and enhance the deposition of the
wafer bumps. The surfactant seems to serve as a mild deposition
inhibitor which can suppress three-dimensional growth to an extent,
thereby improving morphology and topography of the film. It can
also help refine the grain size, which yields a more uniform bump.
Exemplary anionic surfactants include alkyl phosphonates, alkyl
ether phosphates, alkyl sulfates, alkyl ether sulfates, alkyl
sulfonates, alkyl ether sulfonates, carboxylic acid ethers,
carboxylic acid esters, alkyl aryl sulfonates, aryl alkylether
sulfonates, aryl sulfonates, and sulfosuccinates. A particularly
preferred anionic surfactant is Ralufon NAPE 14-90 (available from
Raschig GmbH, Ludwigshafen, Germany) which is a sulfonated
.beta.-naphthol propoxylate/ethoxylate having a block of propylene
oxide units bonded to the .beta.-naphthol hydroxyl group, a block
of ethylene oxide units bonded to the propylene oxide block, and a
terminal propane sulfonate group. Exemplary cationic surfactants
include quaternary ammonium salts such as dodecyl trimethyl
ammonium chloride, cetyl trimethyl ammonium chloride, and the like.
Exemplary non-ionic surfactants include glycol and glycerol esters,
polyethylene glycols, and polypropylene glycol/polyethylene glycol.
Preferred non-ionic surfactants include .beta.-naphthol
derivatives, such as .beta.-naphtholethoxylates having between 1
and about 24 ethylene oxide monomer units bonded to the
.beta.-naphthol hydroxyl group, more preferably between about 8 and
about 16 ethylene oxide monomer units. A particularly preferred
non-ionic surfactant is Lugalvan BNO12 which is a
.beta.-naphtholethoxylate having 12 ethylene oxide monomer units
bonded to the naphthol hydroxyl group. The surfactant can be
present in the electroplating bath at a concentration between about
0.1 g/L and about 50 g/L, preferably between about 5 g/L and about
20 g/L.
The electrolytic plating bath of the present invention preferably
has an acidic pH to inhibit anodic passivation, achieve better
cathodic efficiency, and achieve a more ductile deposit.
Accordingly, the bath pH is preferably between about 0 and about 3.
In the preferred embodiment the pH of the bath is 0. The choice of
acids is limited by the poor solubility or substantial insolubility
of most Ag salts. Accordingly, the preferred acidic pH can be
achieved using nitric acid, acetic acid, and methane sulfonic acid.
In one preferred embodiment, the acid is methane sulfonic acid. The
concentration of the acid is preferably between about 50 g/L and
about 200 g/L, more preferably between about 70 g/L and about 120
g/L. For example, between about 50 g/L and about 160 g/L methane
sulfonic acid can be added to the electroplating bath to achieve a
bath of pH 0 and act as the conductive electrolyte.
The electroplating baths of the present invention are preferably
employed to plate Sn--Ag alloy solder wafer bumps on Cu UBM in the
manufacture of microelectronic devices, such as printed wiring
boards (PWB). However, the plating baths may be used in any
application requiring a Sn-based solder wafer bump. Advantageously,
the plating baths may plate at high rates due to the stability of
the bath and the high current densities which may be applied.
In one application for the invention, the goal of the process is to
yield the product of FIGS. 1 and 2, which is an electronic device
substrate with individual solder bumps thereon. With regard to the
specific process, a first step is substrate preparation. The
substrate preparation is not narrowly germane to the present
invention; but to place the invention in context, it is noted that
the invention falls within the following overall context proceeding
from substrate preparation through inspection: 1) receiving
photoresist patterned wafer with a sputtered Cu seed layer or Cu
UBM (thickness of this layer is normally between 300 .ANG. and 3000
.ANG.) 2) deposit a second UBM layer to prevent diffusion of Cu
into solder bumps such as SnAg (thickness of this layer is normally
1 to 3 microns) 3) deposit solder bumps such as SnAg, normally into
mushroom shape (bump height varies from about 50 microns to 100
microns) 4) strip off photoresist 5) etch off Cu UBM 6) reflow SnAg
bumps 7) inspect and characterization such as bump height and Ag
alloy uniformity measurements, voiding analysis, etc.
During the electrolytic plating operation of the invention for
bumping, the plating solution is preferably maintained at a
temperature between about 15.degree. C. and about 50.degree. C. In
one preferred embodiment, the temperature is between about
25.degree. C. and about 35.degree. C. The substrate is immersed in
or otherwise exposed to the plating bath. The current density
applied is between about 1 A/dm.sup.2 and about 20 A/dm.sup.2,
preferably between about 1 A/dm.sup.2 and about 16 A/dm.sup.2 as
described above. At these current densities, the plating rate is
between about 1 .mu.m/min and about 10 .mu.m/min. Typically, the
thickness of the electrolytically deposited Sn--Ag alloy solder
wafer bump is between about 10 .mu.m and about 100 .mu.m which, in
view of the foregoing plating rates, corresponds to the substrate
being immersed in the solution for a duration between about 10
minutes and about 30 minutes.
The anode may be a soluble anode or insoluble anode. If a soluble
anode is used, the anode preferably comprises Sn(MSA).sub.2, such
that the source of Sn.sup.2+ ions in the plating bath is the
soluble anode. Use of a soluble anode is advantageous because it
allows careful control of the Sn.sup.2+ ion concentration in the
bath, such that the Sn.sup.2+ ion does not become either under- or
over-concentrated. An insoluble anode may be used instead of a
Sn-based soluble anode. Preferable insoluble anodes include Pt/Ti,
Pt/Nb, and DSAs (dimensionally stable anodes). If an insoluble
anode is used, the Sn.sup.2+ ions are introduced as a soluble
Sn.sup.2+ salt.
During the plating operation, Sn.sup.2+ ions and Ag.sup.+ ions are
depleted from the electrolytic plating solution. Rapid depletion
can occur especially with the high current densities achievable
with the plating baths of the present invention. Therefore,
Sn.sup.2+ ions and Ag.sup.+ ions may be replenished according to a
variety of methods. If a Sn-based soluble anode is used, the
Sn.sup.2+ ions are replenished by the dissolution of the anode
during the plating operation. If an insoluble anode is used, the
electrolytic plating solution may be replenished according to
continuous mode plating methods or use-and-dispose plating methods.
In the continuous mode, the same bath volume is used to treat a
large number of substrates. In this mode, reactants must be
periodically replenished, and reaction products accumulate,
necessitating periodic filtering of the plating bath.
Alternatively, the electrolytic plating compositions according to
the present invention are suited for so-called "use-and-dispose"
deposition processes. In the use-and-dispose mode, the plating
composition is used to treat a substrate, and then the bath volume
is directed to a waste stream. Although this latter method may be
more expensive, the use-and-dispose mode requires no metrology;
that is, measuring and adjusting the solution composition to
maintain bath stability is not required.
After electroplating of the SnAg solder bumps is complete, flux is
generously applied with a spray to coat the entire wafer.
Preferably, sufficient flux is applied to impregnate every bump
with flux. After application of flux, the bumps may be reflowed
according to methods known in the art.
Electroplating baths of the present invention are advantageously
stable to spontaneous Ag.sup.+ ion reduction and can be plated at
current densities which are higher than those achievable by
conventional plating baths. The electroplating baths can be used to
plate Sn--Ag alloys having Ag metal content between about 1 wt. %
and about 4 wt. %, preferably between about 2 wt. % and about 3 wt.
%, the process therefore offering the capability to plate Sn--Ag
alloys having a wide range of melting points between about
221.degree. C. and about 226.degree. C. The Sn--Ag solder wafer
bumps are plated with substantially reduced or eliminated voiding
between the bump and the Cu UBM. Moreover, because of the Ag.sup.-
ion stabilizing components of the baths of the present invention,
the Ag metal is distributed uniformly in the Sn--Ag alloys. This
uniformity is important for successful reflow. It also ensures that
the bump properties and mechanical strength are uniform across the
wafer or other substrate.
The following examples further illustrate the present
invention.
Example 1
Sn--Ag Solder Wafer Bump Electroplating Bath and Method of
Preparation
A Sn--Ag bath for electroplating Sn--Ag alloy solder wafer bumps
was prepared comprising the following components: 2.22 g/L HEAT 60
g/L Sn.sup.2+ as 156 g/L Sn(MSA).sub.2 0.5 g/L Ag.sup.+ as 0.94 g/L
Ag(MSA) 100 mL/L MSA (70% solution) 2 g/L Hydroquinone 7.0 g/L of
Lugalvan BNO12 1.6 g/L of Dodigen 226 pH 0
Optionally, the bath may comprise 0.5 grams per liter of Defoamer
SF.
One liter of this bath was prepared according to the following
protocol: 1) Water (about 400 mL) added to a 1 L flask. 2) MSA (100
mL of 70% solution) added and solution stirred. 3) HEAT (2.22 g)
added and solution stirred for about 5 minutes. 4) Ag(MSA) (0.84 g)
added and solution stirred for about 5 minutes. 5) Hydroquinone (2
g) added and solution stirred for about 5 minutes. 6) Sn(MSA).sub.2
(156 g) added and solution stirred for about 5 minutes. 7) Lugalvan
BNO12 (7.0 g) added and solution stirred for about 5 minutes. 8)
With vigorous mixing, Dodigen 226 (1.6 g in aqueous solution) added
dropwise. 9) Add water to 1 L.
This mixing order has been discovered to advantageously result in
increased stability. In particular, mixing the complexing agent and
the Ag source together and providing a period of mixing prior to
adding the Sn source, and prior to adding the surfactant, has been
found to increase stability. It preliminarily appears to provide
advantages in complexing to allow early intimate contact between
the complexor and the Ag.sup.+ ions prior to the introduction of
certain other ingredients. Accordingly, this preferred process
involves combining the source of Ag.sup.+ ions with the complexor
for Ag.sup.+ ions and water to form a bath precursor comprising
Ag.sup.+ ion complex in a substantial absence of the source of
Sn.sup.+2 ions. By "substantial absence" it is meant that 90% or
more of the overall source of Sn.sup.+2 ions is not added until
after intimate mixing of the complexor and the source of Ag.sup.+
ions. It is also seen that in this preferred embodiment, the bath
precursor is substantially free of surfactant, and the process
involves adding the surfactant to the bath precursor after
formation of the Ag complex.
Example 2
Sn--Ag Solder Wafer Bump Electroplating Bath
Another Sn--Ag plating bath was prepared comprising the following
components: 1.75 g/L HEAT 60 g/L Sn.sup.2+ as 156 g/L Sn(MSA).sub.2
0.5 g/L Ag.sup.+ as 0.94 g/L Ag(MSA) 100 mL/L MSA (70% solution) 2
g/L Hydroquinone 7.0 g/L of Lugalvan BNO12 1.6 g/L of Dodigen
226.
Example 3
Sn--Ag Solder Wafer Bump Electroplating Bath
Another bath was prepared comprising the following components: 2.22
g/L HEAT 60 g/L Sn.sup.2+ as 156 g/L Sn(MSA).sub.2 0.5 g/L Ag.sup.+
as 0.94 g/L Ag(MSA) 100 mL/L MSA (70% solution) 2 g/L Hydroquinone
7.0 g/L of Lugalvan BNO12 1.6 g/L of Dodigen 226 0.5 g/L of
Defoamer SF.
Example 4
Sn--Ag Solder Wafer Bump Electroplating Bath
Another bath was prepared comprising the following components: 1.4
g/L HEAT 60 g/L Sn.sup.2+ as 156 g/L Sn(MSA).sub.2 0.5 g/L Ag.sup.+
as 0.94 g/L Ag(MSA) 140 mL/L MSA (70% solution) 2 g/L Hydroquinone
6.7 g/L of Lugalvan BNO12 0.8 g/L of Dodigen 226.
Example 5
Sn--Ag Solder Wafer Bump Electroplating Bath
Another bath was prepared comprising the following components: 8
g/L HEAT 60 g/L Sn.sup.2+ as 156 g/L Sn(MSA).sub.2 0.5 g/L Ag.sup.+
as 0.94 g/L Ag(MSA) 160 mL/L MSA (70% solution) 2 g/L Hydroquinone
8 g/L Ralufon NAPE 1490.
Example 6
Sn--Ag Solder Wafer Bump Electroplating Bath
Another bath was prepared comprising the following components: 6
g/L HEAT 40 g/L Sn.sup.2+ as 104 g/L Sn(MSA).sub.2 0.4 g/L Ag.sup.+
as 0.75 g/L Ag(MSA) 300 mL/L MSA (70% solution) 2 g/L Hydroquinone
8 g/L Ralufon NAPE 1490.
Example 7
Sn--Ag Solder Wafer Bump Electroplating Bath
Another bath was prepared comprising the following components: 3
g/L N,N'-Diethylthiourea (DETU) 60 g/L Sn.sup.2+ as 156 g/L
Sn(MSA).sub.2 0.5 g/L Ag.sup.+ as 0.94 g/L Ag(MSA) 180 mL/L MSA
(70% solution) 2 g/L Hydroquinone 7 g/L Lugalvan BNO 12 2 g/L
Dodigen 226.
Example 8
Sn--Ag Solder Wafer Bump Electroplating Bath
Another bath was prepared comprising the following components: 1.5
g Amidinothiourea 60 g/L Sn.sup.2+ as 156 g/L Sn(MSA).sub.2 1 g/L
Ag.sup.+ as 1.88 g/L Ag(MSA) 180 mL/L MSA (70% solution) 2 g/L
Hydroquinone.
Example 9
Sn--Ag Solder Wafer Bump Electroplating Bath
Another bath was prepared comprising the following components: 3
g/L N,N'-Dimethylthiourea 30 g/L Sn.sup.2+ as 78 g/L Sn(MSA).sub.2
0.3 g/L Ag.sup.+ as 0.56 g/L Ag(MSA) 150 mL/L MSA (70% solution) 2
g/L Hydroquinone 8 g/L Ralufon NAPE 1490.
Example 10
Sn--Ag Solder Wafer Bump Electroplating Bath
Another bath was prepared comprising the following components: 4
g/L N-Allyl-thiourea 60 g/L Sn.sup.2+ as 156 g/L Sn(MSA).sub.2 0.5
g/L Ag.sup.+ as 0.94 g/L Ag(MSA) 100 mL/L MSA (70% solution) 2 g/L
Hydroquinone 7 g/L Lugalvan BNO 12 2 g/L Dodigen 226.
Example 11
Sn--Ag Solder Wafer Bump Deposition
Sn--Ag wafer bumps were plated on Cu UBM using the bath of Example
1. The silicon wafer and Cu UBM die pattern was prepared as
described above. Un-crosslinked photo resistant material was
stripped from the wafer using methylene chloride and ultrasonic
agitation for between 30 and 60 minutes. Cu metallization was
etched by a solution comprising deionized water (60 mL), ammonium
hydroxide (60 mL, 20 to 24% solution), and hydrogen peroxide (10
mL, 3% solution).
Sn--Ag solder wafer bumps were plated from the bath of Example 1
onto dies on patterned silicon wafer. The die positions on the
wafer and bump positions within each die are shown in FIGS. 3A and
3B. The bumps were plated at 12 ASD. Bump height distribution was
measured by surface profilometry (DEKTAK 8000). Table I shows bump
height distribution data obtained from the profilometer. Bump
composition was measured by SEM/EDS (20 kV). Table II shows the Ag
content (in wt. %) data obtained by SEM/EDS. Wafer bump topography
was measured using SEM (20 kV, 45.degree. tilt). FIGS. 1A, 1B, 1C
are SEM photographs showing the Sn--Ag solder wafer bumps as
deposited at 200.times., 800.times., and 3000.times. magnification,
respectively. For each measurement technique, 5 bumps (bump pattern
shown in FIG. 3B and designated 1, 2, 3, 4, and 5 in the horizontal
axes in Tables I and II below) were analyzed at each of 3 die
positions (die pattern shown in FIG. 3A and designated 3, 6, 7 in
the vertical axes in Tables I and II below).
TABLE-US-00001 TABLE I Sn--Ag Solder Bump Height Measurements
Plating Conditions: 12 ASD, 0.8 Hours Measured Bumps (.mu.m) Die
Die No. 1 2 3 4 5 Average 3 64.0 62.0 64.0 62.0 58.0 62.0 6 64.0
63.0 64.0 65.0 58.0 62.8 7 64.0 62.0 64.0 64.0 59.0 62.6 Maxi- mum
Minimum Average Standard Range/ Deviation/ Height Height Range
Height Deviation Average Average (.mu.m) (.mu.m) (.mu.m) (.mu.m)
(.mu.m) (%) (%) 65.0 58.0 7.0 62.5 2.3 11.2 3.7
TABLE-US-00002 TABLE II Sn--Ag Solder Bump Ag Content Measurements
Plating Conditions: 12 ASD, 0.8 Hours Measured Bumps (Ag wt. %) Die
Die No. 1 2 3 4 5 Average 3 2.35 2.26 2.99 3.15 2.52 2.65 6 1.80
2.39 1.78 1.54 2.15 1.93 7 2.85 2.80 2.25 2.25 2.33 2.50 Maxi-
Range Standard mum Minimum (Ag Average Deviation Range/ Deviation/
(Ag wt. (Ag wt. wt. (Ag wt. (Ag wt. Average Average %) %) %) %) %)
(%) (%) 3.15 1.54 1.61 2.36 0.45 68.20 19.22
Example 12
Sn--Ag Solder Wafer Bump Reflow
The Sn--Ag solder wafer bumps of Example 3 were reflowed using a
hot plate in a glove box. Steps: 1. Flux application: Lonco 3355-11
2. Preheat: to evaporate volatiles, allow flux to activate, and
bond to surface 3. Reflow: temperature of hot plate is 260.degree.
C. and nitrogen atmosphere. 4. Cooling: air cooling
Reflowed wafer bumps were photographed using SEM (20 kV, 75.degree.
tilt). FIGS. 2A, 2B, 2C are SEM photographs showing reflowed Sn--Ag
solder wafer bumps as deposited at 200.times., 800.times., and
3000.times. magnification, respectively. The reflowed bumps are
smooth and show a uniform ball shape, and demonstrate zero
voiding.
In view of the above, it will be seen that the several objects of
the invention are achieved and other advantageous results
attained.
When introducing elements of the present invention or the preferred
embodiment(s) thereof, the articles "a", "an", "the" and "said" are
intended to mean that there are one or more of the elements. For
example, that the foregoing description and following claims refer
to "a" wafer bump means that there are one or more such bumps. The
terms "comprising", "including" and "having" are intended to be
inclusive and mean that there may be additional elements other than
the listed elements.
As various changes could be made in the above without departing
from the scope of the invention, it is intended that all matter
contained in the above description and shown in the accompanying
drawings shall be interpreted as illustrative and not in a limiting
sense.
* * * * *